1. Field of the Invention
The invention relates to an MR method which includes the steps of:
generating an enhanced nuclear magnetization in an examination zone under the influence of a first steady magnetic field,
applying a second steady magnetic field to the nuclei previously influenced by the first magnetic field, and
carrying out at least one MR experiment after the first steady magnetic field has ceased to act on the nuclei but before the nuclear magnetization influenced by this magnetic field has decayed, the MR experiment including the excitation of the nuclear magnetization by means of an MR RF magnetic field under the influence of the second steady magnetic field. The invention also relates to a device for carrying out such a method.
2. Description of Related Art
A method of the kind set forth is already known from a publication by Baras et al. in Proceedings SMR/ESRMB, Nice, 1995, Vol. 2, page 296. According to the known method first an enhanced nuclear magnetization (or magnetic polarization) is generated by means of the first steady magnetic field. This magnetic field is only briefly switched on (therefore, in the context of the present invention the term xe2x80x9csteadyxe2x80x9d is to be broadly interpreted; a magnetic field which has a steady value for only a few tenth of a second is also xe2x80x9csteadyxe2x80x9d in this sense). The first steady magnetic field must be very strong, but the requirements imposed as regards its uniformity are very mild. Subsequently, a second steady magnetic field acts on the nuclei influenced by the first magnetic field, an MR experiment then being performed in the normal manner. The second steady magnetic field may be significantly weaker than the first field, but requires a homogeneity which suffices for the execution of MR examinations.
The enhanced nuclear magnetization realized by the first steady magnetic field continues to exist for some time after the deactivation of this field, and the associated enhanced signal-to-noise ratio can be used when the MR experiment(s) is/are carried out under the influence of the second steady magnetic field within a short distance in time from the first steady magnetic field.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant""s invention of the invention subsequently claimed.
It is an object of the present invention to improve a method of the kind set forth even further. This object is achieved according to the invention in that the directions of the first and the second steady magnetic field enclose an angle other than 0xc2x0, preferably 90xc2x0, relative to one another, and in that the two steady magnetic fields overlap in time.
As opposed to the cited known MR method, in which the two steady fields have the same direction, the directions of these two fields according to the invention enclose an angle other than 0xc2x0, preferably an angle of 90xc2x0. It is to be noted that the publication by Conolly et al in Proceedings SMRM, New York, 1993, Vol. 3, page 1365 already describes a method in which first a strong magnetic field is switched on and off in the x direction and subsequently a weak magnetic field which extends and oscillates in the z direction is activated, thus causing the vector of the nuclear magnetization, previously extending in the z direction, to be rotated in the x, y plane in conformity with the Larmor frequency.
The invention, however, is based on the recognition of the fact that when the nuclear magnetization comes under the influence of the second steady magnetic field while the first steady magnetic field is still effective and subsequently disappears, the nuclear magnetization vector is rotated from the direction imposed by the first magnetic field to the direction imposed by the second magnetic field.
The fact that the vector of the nuclear magnetization thus retains (for the time being) its value enhanced by the first steady magnetic field but is oriented in conformity with the second steady magnetic field can be utilized in various ways so as to improve MR examinations. In a first, preferred possibility, the generating of the enhanced nuclear magnetization includes the excitation of the electron spin resonance of a contrast agent, present in the object to be examined and containing un-paired electrons under the influence of the first steady magnetic field, by an ESR RF magnetic field, such that the directions of the ESR RF magnetic field and the MR RF magnetic field enclose an angle of at least approximately 90xc2x0 relative to one another. In that case the nuclear magnetization is not enhanced by a particularly strong first steady magnetic field, but by the fact that, in conjunction with a (comparatively weak) magnetic field, the electron spin resonance (ESR) of a contrast agent which is present in the object to be examined and contains un-paired electrons is excited by an ESR RF magnetic field. The excitation of the electron spin resonance is then transferred to the nuclei and hence enhances the nuclear magnetization; this effect is generally referred to as the Overhauser effect. The subsequent excitation of the nuclear magnetization by an MR RF magnetic field in conjunction with a second (again comparatively weak) steady magnetic field produces MR signals with a signal-to-noise ratio of a quality that otherwise can be achieved only by means of significantly stronger magnetic fields.
In a further version, the first steady magnetic field is weaker than the second steady magnetic field. This version offers the advantage that the ESR RF magnetic field can still penetrate the examination zone sufficiently far and that an improved signal-to-noise ratio is obtained because of the stronger second steady magnetic field.
A device for carrying out the methods of this invention includes a magnet for generating a first steady magnetic field, an ESR RF coil system for generating an ESR RF magnetic field which extends at least substantially perpendicularly to the first steady magnetic field, and an MR RF coil system for generating and/or receiving an MR RF magnetic field, and is characterized in that there is provided a second magnet for generating a second steady magnetic field whose direction encloses an angle other than 0xc2x0, preferably 90xc2x0, relative to the direction of the first steady magnetic field, and in that the direction of the MR RF magnetic field extends at least substantially perpendicularly to the direction of the second steady magnetic field and to the direction of the ESR RF magnetic field. This device based on the following considerations: the frequency of the ESR RF magnetic field is a factor of approximately 660 greater than the frequency of the MR RF magnetic field. The ESR and the MR RF magnetic fields, therefore, must be generated by means of separate RF coils. If the RF fields were oriented in the same direction (which would be necessary if the two steady magnetic fields both extend in the same direction), strong interaction could occur between the coils. However, because the two steady magnetic fields extend perpendicularly to one another, the two RF fields, being temporally linked to a respective one of these fields, may also extend perpendicularly to one another, interaction between the RF coils generating these fields being avoided.
Therefore, it is also possible to use two coaxial RF coils (of a different type), which enclose one another and which enclose the examination zone. In the case of parallel oriented ESR and MR RF magnetic fields such a construction would either give rise to excessive interactions between the coils or, if the diameter of the MR coil were significantly greater than that of the ESR coil, to a drastically reduced signal-to-noise ratio. When the MR RF coil and the ESR RF coil in the device according to the invention very tightly enclose one another and the examination zone, a suitable signal-to-noise ratio will be obtained, because the coils do not influence one another, despite the fact that they are arranged so near one another.
In a further embodiment the MR RF coil includes a solenoid coil. A solenoid coil generates a magnetic field which extends in the direction of its axis. Even though there are also other coils generating a magnetic field in this direction, for example coils consisting of a plurality of circular conductor loops which are only inductively coupled to one another, a solenoid coil offers advantages in respect of the signal-to-noise ratio at low frequencies. Attractive embodiments of the ESR RF coil system include a quadrature coil of the bird-cage type, a TEM resonator, or an array of surface coils enclosing a cylindrical space. Such RF coil systems all generate RF magnetic fields extending perpendicularly to the coil axis.
In a further embodiment of the methods of this invention, the first steady magnetic field is switched on and off in a pulsed manner, is stronger than the second steady magnetic field, and the homogeneity of the second steady magnetic field is greater than that of the first steady magnetic field.
In this embodiment the enhanced nuclear magnetization is obtained in that the first steady magnetic field is significantly stronger than the second steady magnetic field. When the second steady magnetic field extends in the longitudinal direction of the examination zone,.the first steady magnetic field can be generated with the necessary strength by means of a comparatively simple coil.
In a further embodiment of the invention, the first steady magnetic field acts on a first part of the patient and the second steady magnetic field acts on a second part of the patient, and the blood of the patient flows from the first part to the second part. It utilizes the fact that the nuclear magnetization enhanced by the first magnetic field is transported, via the blood, to the second part where the second steady magnetic field acts on the nuclei influenced by the first magnetic field.